The present invention relates to electroconductive iron oxide particles having excellent stability and electroconductivity, and assuming a reddish to dark purple, liver brown or golden yellow color, and a process for producing such electroconductive iron oxide particles. More particularly, the present invention relates to electroconductive iron oxide particles having a volume resistivity of not more than 5.times.10.sup.6 .OMEGA.-cm, and comprising iron oxide particles selected from hematite and maghemite, and SnO.sub.2 particles in which Sb is contained as a solid solution deposited on the surfaces of the iron oxide particles, and a process for producing such electroconductive iron oxide particles which comprises stirring an aqueous suspension containing iron oxide particles selected from hematite and maghemite, and Sb-containing SnO.sub.2 hydrate (SnO.sub.2 .multidot.2H.sub.2 O) [hereinafter referred to as SnO.sub.2 hydrate] particles so that the Sb-containing SnO.sub.2 hydrate particles are deposited on the surface of the iron oxide particles, subjecting the thus deposited iron oxide particles to filtration, washing with water and drying, and then calcining the thus obtained iron oxide particles at a temperature of 400.degree. to 800.degree. C.
The electroconductive iron oxide particles according to the present invention are particularly useful as an antistatic material.
Recently, use of a clean room is prevailing for the assurance of safety, hygienic working atmosphere and precision of works In order to minimize dust in such clean room, antistatic materials are being used for the room. Application of antistatic materials is also required for preventing discharge break of IC's and LSI's
Generally, an antistatic material is produced by dispersing the particles of an electroconductive substance in a base material such as coating material, rubber, plastic, etc., to afford electroconductivity to such material In the production of such antistatic material, it is necessary that the electroconductive substance applied on the base material does not lower the properties of the base material. This requires that the particles of the electroconductive substance to be dispersed in the base material have per se high electroconductivity so that they can afford the desired conductivity to the base material with as small a content as possible. This fact is described in many literatures, for example, "The 24th Pigment Engineering Seminar--Conductive materials and Their Applications" (1986) held under the joint auspices of the Kanto branch of Journal of the Japan Society of Colour Material, the Pigment Technology Research Society and the Kanto branch of the Japan Pigment Technology Association, which states on pages 1-19: " . . . the properties required for the conductive fillers are shown in FIG. 2. Needless to say, there are required those fillers which show the desired conductivity with a small content and deteriorate the resin properties."
The electroconductive material particles are also required to be highly resistant to the acid(s) in the vehicle in the production of coating material, etc., and to have excellent weather resistance so as not to cause environmental pollution. This fact is also described in the above-mentioned literature, "the properties, . . . , stability, . . . of the preferred fillers" with reference to "FIG. 2".
On the other hand, the improvements of cultural and social life standards in the recent years have raised the requirements for not only the improvement of functional qualities of commodities but also greater variety of their appearance from the sensory and fashionable aspects. Concerning color, for instance, there is strong request for a larger variety of colors, especially for the beautiful chromatic colors.
Various kinds of conductive material particles are now available. For instance, carbon particles such as a carbon black powder, metal particles such as a copper powder, an aluminum powder, a nickel powder, etc., reduced titanium oxide particles, mica particles coated with Sb-containing SnO.sub.2 or TiO.sub.2, metal oxide particles such as titanium oxide particles coated with SnO.sub.2 containing Sb as a solid solution are known.
The conductive material particles having excellent stability and electroconductivity are the most strongly required at present. However, the presently available carbon particles, although excellent in stability, have the problems in the esthetic aspect because of their black color. While the metal particles obtainable from the conventional methods, although having excellent electroconductivity, are low in resistance to the acid(s) in the vehicle and also poor in weather resistance and stability. Further, the metal oxide particles, although high in stability, are unsatisfactory in electroconductivity. For example, the electric resistance of the iron oxide particles, which are the most typical example of the metal oxide pigment particles, is on the order of 10.sup.8 to 10.sup.9 .OMEGA.-cm It is commonly practiced to reduce the metal oxide particles or coat the metal oxide particles with various kinds of material for giving electroconductivity to such particles. However, all of the available metal oxide type conductive material particles are white or of an achromatic color ranging from gray to black.
Thus, the estabilishment of technical means for obtaining the electroconductive material particles having excellent stability and electroconductivity as well as a beautiful chromatic color other than black color has been strongly required.
As a result of the extensive studies for obtaining the conductive material particles having excellent stability and electroconductivity as well as a beautiful chromatic color, it has been found that by stirring and mixing up an aqueous suspension containing iron oxide particles selected from hematite and maghemite, and the Sb-containing SnO.sub.2 hydrate particles so that said Sb-containing SnO.sub.2 hydrate particles are deposited on the surfaces of said iron oxide particles, subjecting these iron oxide particles to filtration, washing with water and drying, and then calcining the thus obtained iron oxide particles at a temperature in the range of 400.degree. to 800.degree. C., thereby obtaining electroconductive iron oxide particles having a volume resistivity of not more than 5.times.10.sup.6 .OMEGA.-cm, and comprising iron oxide particles selected from hematite and maghemite, and SnO.sub.2 particles in which Sb is contained as a solid solution deposited on the surface of the iron oxide particles. The present invention was attained on the basis of such finding.